View Full Version : gears & clearances?

09-20-2003, 06:37 AM
greets all.
i was wondering how much, if any, clearance i need between two gears and how to measure?

i have a pinion carrying a load across a (horizontal) rack. as it is, the pinions are also the "wheels" -- if that makes sense.. they hold the full weight of the load, and move it left to right.

basically, i've got 100% meshing of the two gears (rack and pinion).. is this an okay design? or should be using some sort of bearing(s) to hold the weight, and the pinions only doing the moving? that is, should i leave a few thou clearance between the two?

(if i dont, what happens? i imagine it would be premature wear, if anything, but nothing as drastic as seizure)


09-20-2003, 09:08 AM
Gears are designed for rolling contact on the teeth as the teeth mesh. If the gears are transmitting torque, only one side of the tooth will be in contact.

If the gears are forced together as you describe, both sides of the teeth will be in contact. That means at least one will be rubbing instead of rolling.

I'd use bearings to carry the load, and let the gear teeth work like they're designed to work. Of course, if the load is light, the speed is low, the parts get cleaned/greased regularly, and you aren't concerned about the wear, it'll probably work fine for a long time without bearings.

Eventually, the wear may let the teeth "wedge" into one another under load, making travel difficult.

Al Messer
09-20-2003, 10:09 AM
Use a piece of notebook paper to set the clearance between the gears. I do it all the time when setting up a new gear train in my old Montgomery Ward lathe.

Forrest Addy
09-20-2003, 10:29 AM
A gear set should never mesh metal to metal or bear a direct radial load. This would force the teeth to wedge. Therefore a small separation has to be maintained between meshing teeth and this is measured by at the space between them, usually by eye once skill is developed. This separation or clearance is called "backlash" in gears.

Backlash in fishing reels is bad but good in gears.

There's an algorithm for setting backlash in general purpose gearing. Generally, 40 divided by diametral pitch gives you the backlash in thousandths of an inch.

For example a 10 diametral pitch gear pair will work out to 40/10. Applying the rule: the result equals the number of thousandths reccommended backlash. In this example it would be 0.004".

This algorithm will have to be converted if you're working with metric units.

As I said, it's an easily remembered algorithm intended for rough and ready mental calculation not an exact formula. It only gives you a raw number and you have to apply a rule to place the decimal place, determine the units, etc.

[This message has been edited by Forrest Addy (edited 09-20-2003).]

09-20-2003, 12:19 PM
Deformable strips (plastic, I think) are available for this purpose. Rotate the gears while inserting an appropriately sized piece into the mesh, then remove and measure the strip's new thickness.

Dunno how the material avoids "springback" but it's veen available and used for years. Indium foil also works.

I'm not familiar with Forrest's rule of thumb but it sounds practical.

09-20-2003, 11:08 PM
Sounds to me like a very low speed meshing, and low duty cycle application. If so, I think it's fine the way it is. The wear will be there, but the lifetime could be very long anyway, and adding bearings could be overkill. One advantage to you could actually be the lack of play in these teeth, depends on the mechanism.

09-21-2003, 07:21 PM
thanks for all the info.
i was hoping it was something i could "get away with" ... but binding the gears is the last thing i want to do.

the device is essentially a conveyor.
moving about 800 pounds (load + its own weight) ... about 20 feet.. maybe 2 or 3 times a day. (i guess that would be 4 or 6, considering the round-trip).

two bearings, that will hold this load, will run me about $150.

not that i mind spending the money, if it is a job better-done.. but i didnt want to overdesign/overkill the machine.


09-21-2003, 07:32 PM
You will proably break some teeth unless you relieve the load on the rack with bearings. The wedging action of 800Lbs. may well pop some teeth on the rack. For reliablility bearings are a must.